How a Motorcycle Works
by Henry Pasternack
From: henry AT ginger.sri.com (Henry “Credible” Pasternack)
Subject: How a motorcycle works.
Date: 29 Nov 89 18:49:01 GMT
Chapter 1: The Engine.
The engine has many components, each of which is required for the proper functioning of the whole. When the crankshaft spins, the offset lobes carrying the journals set up harmonic vibrations in the cases and cylinders. These vibrations cause air to be sucked out of the combustions chambers, past the rings which act like one-way valves in much the same manner as the reed valve in a diaphragm pump. The resulting vacuum causes the intake and exhaust valves to open (due to the pressure differential) allowing mixture to be drawn into the cylinder on the carb side, and spent combustion fumes on the exhaust side. The momentum of the inrushing gas pushes the pistons down, in what is known as the “intake stroke”. The connecting rods prevent the pistons from falling out of the cylinders and into the cases, or from hitting the combustion chamber sides.
After the chamber is full, the pressure differential holding open the valves is relieved, and the valves shut. The mixture begins to cool slightly, producing a reverse pressure gradient, owing to the large amount of air pumped into the crankcase. Along with the vibratory energy supplied by the crankshaft, the pistons begin to move back towards the tops of the cylinders in what is known as the “compression stroke”. As the density of the cylinder gases increases, the air and gasoline molecules are forced together more and more. Because they are already quite hot from spent gases which were taken in during the intake cycle, their kinetic energy is high, and many collisions occur. At a critical point, the air and gas molecules suddenly combine violently. The “combustion” causes the mixture to implode, and cool rapidly as the kinetic and thermal energy is absorbed. This makes the gas pressure drop, and the pistons are sucked strongly towards the top of he cylinders. The resulting unbalanced acceleration produces a driving force which is resonant with the vibration of the crankshaft, reinforcing its motion.
Because of the momentum imparted to the pistons during the implosion of the fuel/air mixture, they are now moving quite rapidly. As they are constrained by the connecting rods, they rebound at the top of the compression stroke, before hitting the cylinder head. The wrist pins and rod bearings are made of very hard metal, so the collision is very nearly elastic, and no energy is lost. Consequently, the pistons rebound, and begin traveling downward with an equal, but opposite velocity. The cylinder volumes thus begin to increase, and the gas pressure drops even lower than it was after implosion. This is called the “power stroke” because the motion of the pistons at this point is more powerful than it is at any other time in the cycle.
This extreme cylinder vacuum causes the intake valves to open, because the intake ports are maintained at atmospheric pressure. The exhaust valves, however, stay closed because of a vacuum which has developed in the headers. The cause of this vacuum is several fold: First, a rarefaction was caused during the intake stroke when hot exhaust gases were sucked into the cylinders. Second, the heat sinking effect of the long mufflers (whose job it is to cool the exhaust in order to most efficiently extract spent implosion products) has caused the exhaust to cool during the intake/compression cycle. Third, the acoustical resonance of the pipes reinforces the vacuum pulses at certain frequencies. This is why bikes have “powerbands”.
When the intake valves open, fresh mixture rushes into the cylinders, immediately neutralizing the vacuum due to implosion. This allows the pistons to move full speed to the bottoms of the cylinders where they again rebound off the connecting rods and begin traveling upwards. The gas is squeezed and pressure rises, creating a superheated condition in the spent gases. The pressure is relieved by the piston rings and vented into the crankcase, where the gases are safely relieved to the exhaust pipe by the crankcase breather and exhaust gas recirculation system. This is called the “exhaust stroke”.
Most engines are of the type known as “Diesel”, named after Dr. Mercedes Diesel, who invented the internal combustion engine in Germany at the turn of the century. Diesels have the advantage of being able to run on relatively crude fuel. They last a long time because they are simple. There is another kind of engine, called the “Otto” engine, which burns gasoline. The “Otto” engine has several extra parts, namely a camshaft, ignition system, and spark plugs. The camshaft has eccentric lobes which are acted upon by the movement of the valves, causing the shaft to spin. The spinning shaft is used for timing the ignition system which periodically sends a burst of high voltage to the spark plugs located in the top of each cylinder. The spark comes at the end of the exhaust stroke, adding additional heat to the last portions of the spent implosion gases. This improves exhaust scavenging, greatly increasing the power and efficiency of the engine.
Next chapter: The Transmission.